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Does the Length of the Tube Make a Difference in Image Quality?

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Old Monday 17th December 2018, 22:05   #1
MUHerd
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Does the Length of the Tube Make a Difference in Image Quality?

Hey all,

quick Question, if you don't mind.

I am looking into getting a new/better set of Binos and getting back outdoors to enjoy nature again.

One thing that I am clueless about is whether the length of the "tube" has any effect on image quality.

Lets say that I'm looking at 2 sets of binos, each are 10x42, but one of them is more compact than the other. Maybe the compact is 1 to 2 inches shorter than the other, will that difference in length have any effect on the quality of the image produced? Assume that the glass in both are ED.

Would the longer produce the best image?

Is there any kind of "rule of thumb" type thing regarding the length of the tubes on binos and image quality?

Thank you all kindly.

Larry
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Old Monday 17th December 2018, 23:57   #2
tenex
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In general, shorter binos may be compromising in various ways to acheive compactness: higher aberrations due to shorter focal length objectives, narrower FOV, etc. Then again, when Leica does this the results are still pretty nice, so you'll have to check specific models of interest to you.
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Old Tuesday 18th December 2018, 08:41   #3
jring
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Hi,

generally for telescopes of a given aperture a design with a longer focal length and thus a more relaxed focal ratio will be more forgiving as in not showing aberrations so easily. Very fast designs (f5 and below) tend to need a triplet objective lens (or a very experienced mirror maker being able to create a strong parabolic surface to very exacting standards) to work well.

Most current binoculars have doublet objective lenses with a very fast focal ratio around f4 - some with an ED element, but usually the magnifications are low enough so the designer gets away with it.

Joachim
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Old Tuesday 18th December 2018, 13:36   #4
henry link
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As Joachim said, given identical designs including glass types, the focal ratio of the objective is what determines the levels of chromatic and spherical aberrations.

In the case of binoculars the EFFECTIVE focal ratios usually can't be known and certainly can't be determined by simply measuring the physical length. That's because the exact designs and glass types vary a lot from simple cemented crown/flint doublets to fixed air-spaced ED triplets combined with moving focusing singlets or occasionally doublets. Some use positive focusing lenses and some negative to arrive at the final effective focal ratio which can be above f/4 or approach f/3. In such complex objective designs the physical length has no direct relationship to the effective optical focal ratio. In addition prism type and size also affects the physical length without affecting the optical focal ratio. Besides all that some objective designs require a higher focal ratio just to equal the aberrations of other designs.

Even with all those variables there is a way to increase the probability of experiencing the advantages of a high focal ratio and that is to choose a high quality large exit pupil binocular. I use a Zeiss 8x56 FL, which has a pretty high aberration objective lens of, I think, around f/3.2-3.4. However, at typical daylight birding light levels the eye stops down its objective to a low aberration f/7-f/9, which results in a cleaner and more transparent image than I've seen in any smaller exit pupil binocular.

Last edited by henry link : Tuesday 18th December 2018 at 17:23.
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Old Tuesday 18th December 2018, 18:46   #5
WJC
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Quote:
Originally Posted by MUHerd View Post
Hey all,

quick Question, if you don't mind.

I am looking into getting a new/better set of Binos and getting back outdoors to enjoy nature again.

One thing that I am clueless about is whether the length of the "tube" has any effect on image quality.

Lets say that I'm looking at 2 sets of binos, each are 10x42, but one of them is more compact than the other. Maybe the compact is 1 to 2 inches shorter than the other, will that difference in length have any effect on the quality of the image produced? Assume that the glass in both are ED.

Would the longer produce the best image?

Is there any kind of "rule of thumb" type thing regarding the length of the tubes on binos and image quality?

Thank you all kindly.

Larry
As optical scripture says: “A longer focal length hides a multitude of sins.” In this case, those sins are third-order aberrations.

By the way, I have seen a number of non-ED binos that were head and shoulders better than others sold as ED instruments. To learn what matters and what doesn’t the inquisitor must either learn something about optics or believe whatever lie that comes along.

Bill
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Old Sunday 23rd December 2018, 22:50   #6
justabirdwatcher
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By the way, I have seen a number of non-ED binos that were head and shoulders better than others sold as ED instruments. To learn what matters and what doesn’t the inquisitor must either learn something about optics or believe whatever lie that comes along.

Bill
Or, just try a crap-ton of bins and see for themselves.
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